1,967 research outputs found
Validity of the Gor'kov expansion near the upper critical field in type II superconductors
We have examined the validity of the Gor'kov expansion in the strength of the
order parameter of type II superconductors near the upper critical field.
Although the degeneracy of the electron levels in a magnetic field gives non-
perturbative terms in the solution to the Bogoliubov-de Gennes equations we
find, contrary to recent claims, that these non-perturbative terms cancel in
the expression for the thermodynamic potential, and that the traditional
Gor'kov theory is correct sufficiently close to Hc2 at finite temperature. We
have derived conditions for the validity of the Gor'kov theory which
essentially state, that the change in the quasiparticle energies as compared to
the normal state energies cannot be too large compared to the temperature.Comment: 5 pages, 3 figures. One reference adde
Frequency and damping of the Scissors Mode of a Fermi gas
We calculate the frequency and damping of the scissors mode in a classical
gas as a function of temperature and coupling strength. Our results show good
agreement with the main features observed in recent measurements of the
scissors mode in an ultracold gas of Li atoms. The comparison between
theory and experiment involves no fitting parameters and thus allows an
identification of non-classical effects at and near the unitarity limit.Comment: 4 pages, 2 figure
Two-component Fermi gas with a resonant interaction
We consider a two-component Fermi gas interacting via a Feshbach molecular
state. It is shown that an important energy scale is
where is the Feshbach coupling constant and the mass of the particles.
Only when where is the Fermi
energy can the gas be expected to enter a universal state in the unitarity
limit on the atomic side of the resonance where there are no molecules present.
The universal state is distinct from the molecular gas state on the other side
of the resonance. We furthermore calculate the energy of the gas for this
universal state and our results are related to current experiments on Li
and K.Comment: 4 pages, 2 figure
Magnetic and superfluid phases of confined fermions in two-dimensional optical lattices
We examine antiferromagnetic and d-wave superfluid phases of cold fermionic
atoms with repulsive interactions in a two-dimensional optical lattice combined
with a harmonic trapping potential. For experimentally realistic parameters,
the trapping potential leads to the coexistence of magnetic and superfluid
ordered phases with the normal phase. We study the intriguing shell structures
arising from the competition between the magnetic and superfluid order as a
function of the filling fraction. In certain cases antiferromagnetism induce
superfluidity by charge redistributions. We furthermore demonstrate how these
shell structures can be detected as distinct anti-bunching dips and pairing
peaks in the density-density correlation function probed in expansion
experiments.Comment: 4 pages, 3 figure
Feshbach Resonances and Medium Effects in ultracold atomic Gases
We develop an effective low energy theory for multi-channel scattering of
cold atomic alkali atoms with particular focus on Feshbach resonances. The
scattering matrix is expressed in terms of observables only and the theory
allows for the inclusion of many-body effects both in the open and in the
closed channels.
We then consider the frequency and damping of collective modes for Fermi
gases and demonstrate how medium effects significantly increase the scattering
rate determining the nature of the modes. Our results obtained with no fitting
parameters are shown to compare well with experimental data.Comment: Presented at the 5th workshop on Critical Stability, Erice, Italy
13-17 October 2008. 8 pages, 3 figures. Figure caption correcte
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